A parallel plate type (capacitively coupled) plasma processing apparatus, an inductively coupled plasma processing apparatus, a microwave plasma processing apparatus or the like has been widely used for performing microprocessing such as etching or film forming process on a processing target object by plasma. Among these apparatuses, in the parallel plate type plasma processing apparatus, a high frequency power is applied to at least one of an upper electrode and a lower electrode facing the upper electrode. As a result, a gas is excited into plasma by electric field energy of the high frequency power, and the microprocessing is performed on the processing target object by the generated electric discharge plasma.
To meet a recent requirement for a higher level of miniaturization, it has become necessary to supply an electric power of a high frequency in a relatively high frequency range of about 100 MHz and generate plasma having high density and low ion energy. If the frequency of the supplied electric power increases, a high frequency current flows along a surface of the (upper or lower) electrode facing the plasma (hereinafter, referred to as a “plasma facing surface”) from an edge portion of the electrode toward a central portion thereof by a skin effect. Accordingly, electric field intensity at the central portion of the electrode becomes higher than electric field intensity at the edge portion thereof. As a result, electric field energy consumed to generate plasma at the central portion of the electrode becomes higher than electric field energy consumed to generate plasma at the edge portion of the electrode. Therefore, ionization or dissociation of a gas is accelerated at the central portion of the electrode. Consequently, electron density of the plasma at the central portion of the electrode becomes higher than electron density of the plasma at the edge portion thereof. Since electrical resistivity of plasma is decreased at the central portion of the electrode where the electron density of plasma is high, an electric current caused by high frequency wave (electromagnetic wave) is also concentrated at a central portion of a facing electrode. Therefore, plasma density becomes non-uniform therein.
To solve this problem, there has been proposed burying a dielectric member such as ceramic in a central portion of the plasma facing surface of the electrode in order to improve uniformity of plasma density (see, for example, Patent Document 1).    Patent Document 1: Japanese Patent Laid-open Publication No. 2000-323456
If, however, the dielectric member such as ceramic is buried in the electrode, a stress may be applied to a joint portion between the electrode and the dielectric member due to a difference in their thermal expansion caused by repetitive heating and cooling during a process. As a result, the dielectric member may crack or the inside of a chamber may be contaminated.
In order to further improve plasma uniformity, it has been also proposed to form the dielectric member, which is buried in the electrode, in a tapered shape. In such a case, since a capacitance component at an edge portion of the dielectric member becomes larger than a capacitance component at a central portion thereof, a decrease in electric field intensity at the edge portion of the dielectric member is not great compared to a case of burying a dielectric member having a flat shape in the electrode. Thus, more uniform electric field intensity can be obtained.
If, however, the dielectric member is formed in the tapered shape, dimensional accuracy of a tapered portion may be deteriorated depending on machining accuracy. As a consequence, a stress may be concentrated on the tapered portion of the dielectric member due to a thermal expansion difference or non-uniformity in dimensional tolerance at a joint interface. Due to the stress concentration, a crack of the electrode is more likely to occur, and the inside of the chamber may be easily contaminated with particles or metals.